The invention relates to a single lens having two aspherical refractive surfaces. The lens comprises a lens substrate and two layers of transparent plastic having aspherical outer profiles on opposite sides of the substrate.
U.S. Pat. No. 4,027,952 describes a class of lenses having two aspherical surfaces, also referred to as biaspherical lenses. Biaspherical lenses may be suitable for use as objectives in devices for reading optical record carriers containing, for example, video programs or audio programs.
The small dimensions, on the order of 1 .mu.m, of the information carrying structure on these record carriers impose stringent requirements on the objectives. For example, such objectives must be capable of forming diffraction-limited radiation spots having a half-intensity diameter on the order of 1 .mu.m. This means that for the customary wavelength of the read beam, the objective must have a numerical aperture of approximately 0.45. Further, this objective should have a comparatively large diffraction-limited image field on the order of a few hundreds of microns.
Currently, the read devices for optical record carriers are equipped with an objective system which comprises at least three lens components which all have spherical surfaces.
As stated in U.S. Pat. No. 4,027,952 such a composite and expensive objective system can be replaced by a single biaspherical lens. Such a lens can be manufactured simply and cheaply in large quantities using a so-called replication process. This process starts with a substrate, also referred to as preform, of a transparent and stable material such as glass. Both the object side and the image side of the preform are provided with a plastic layer which is in a more or less liquid condition. The plastic may be, for example, an ultraviolet-curable polymerizable material. Dies are pressed onto the plastic layer. The inner surfaces of the dies have shapes which are negatives of the desired aspherical shapes. After curing the layers, for example by exposure to ultraviolet light, the dies are removed and the desired lens is obtained.
To date the manufacture of a biaspherical lens starts with a preform having two spherical surfaces. In order to obtain a maximum image field, the distance between these surfaces, measured along the optical axis, is smaller than the sum of the radii of curvature of the surfaces.
The manufacture of such a preform is rather time consuming and expensive, so that the cost of the resulting biaspherical lens, which is mainly determined by the cost of the preform, is high. It has also been proposed to manufacture biaspherical lenses from preforms already having a certain degree of asphericity. However, the manufacture of such a preform is even more expensive.
Efforts continue to simply and inexpensively mass manufacture devices for reading optical record carriers containing, for example, video programs or audio programs. An important aspect of this is to make the objective system simpler and cheaper without degrading the readout quality.